1. Field of the Invention
The present invention relates to an optical communication system capable of performing bidirectional data communications through a single transmission line between two distant places.
2. Description of the Prior Art
In single line bidirectional communication using the single transmission line, e.g., one optical fiber, the prior art is accompanied by the problem of a crosstalk in which an optical signal transmitted by itself is mixed into the optical signal from a remote side. This problem is caused, in a multiple optical wavelength system changing the mutual optical communication wavelengths and separating them by using a branching filter, by the crosstalk due to insufficient separation between the optical wavelengths and, in an identical optical wavelength system using a directional coupler, by the crosstalk due to a reflected beam at the connected point of the optical fiber.
As a countermeasure, it is conceived in the prior art to sufficiently widen the interval between the individual wavelengths in the multiple optical wavelength system. As a result, the prior art is defective in a remarkably high cost and is impractical because of the use of a light emitting element of special wavelength and because of the requirement of the branching filter to be made highly precisely. In the identical optical wavelength system, on the other hand, the influences of the reflected beam are prevented by setting the threshold voltage Vth of the optical receiving circuit at a higher level than that of the reflected beam intensity by means of a half-fixed resistor or a changeover switch. This system is constructed at a lower cost because it can be constructed of an optical transmitting circuit of the same wavelength and of the directional coupler, but has to have the threshold voltage adjusted, each time the system is installed, so as not to detect the reflected beam but to detect only the signal beam because the threshold voltage Vth is fixed. In the case of a long-range communication in which the signal beam becomes weak, moreover, the difference between the signal voltage and the threshold voltage becomes so small as to facilitate degradation of the bit error rate (i.e., BER) so that the system can be used only for a short-range communication. Still moreover, the directional coupler of the prior art cannot eliminate the near-end reflection, because it uses a half mirror, so that it makes impossible the long-range communication in which the signal beam becomes weak.
Furthermore, in case the power supply of the remote communication system is OFF or in case the optical fiber is not connected or cut, the single line bidirectional communication of the prior art is accompanied by a problem that the intra-office station will detect the carrier outputted from itself, when its power supply is ON, to judge as if the communication circuit would be set. This makes it necessary for the user to confirm every time whether or not the circuit is really set. Moreover, the abnormality of the line in operation cannot be easily detected for the same reason.